Kenney is a Dutch game developer who created this giant asset resource back in 2010 and has been updating it with new content ever since. They specialize in low- poly games and even have a 2D sprite editor called Kenny Studio.
All their free assets are distributed with a public domain license, so you can even use them for commercial purposes. And the collection is huge, with over 40 thousand images, audio files, and 3D models.
Kenney also has pixeland, which they call the world’s first “indie game experience center”. Basically, it is an online community for game devs, hobbyists, students, etc. to share ideas and artwork.